Filter



Patented Oct. 15, 1946 FILTER Ralph lW. George, Riverhead, N. Y.,assignor to Radio Corporation of America, a corporation of DelawareApplication October 12, 1942, Serial No. 461,715

(Cl. 17g- 44) Claims.

The present invention relates -to filter networks and, moreparticularly, to filters especially adapted for use with ultra highfrequencies.

An object of the present invention is the provision of a band rejectionlter which is suitable for ultra high frequencies.

Another object of the present invention is the provision of a filterwhich can be made to have a high attenuation over a wide range offrequencies.

Still another object of the present invention is the provision of afilter, as set forth above, which has a high current carrying capacity.

A further object of the present invention is the provision of a filter,as set forth above, which is simple in mechanical construction.

Still a further object of the present invention is the provision ofmeans for preventing leakage of ultra high frequency energy from asignal generator along potential supply and control leads.

The foregoing objects, and others which may Yappear from the followingdetailed description,

are attained by the provision of a lter in the form of a coaxialtransmission line wherein the outer conductor is grounded and the innerconductor is by-passed to the outer conductor at predetermined pointsalong its length. The inner conductor may be made large enough to safelycarry any desired value of low frequency current. Furthermore, the innerconductor may be made in the form of a control shaft for operatingtuning condensers and the like within the shielded Ycase of a signalgenerator.

rThe novel features which, it is believed, are characteristic of thepresent invention are pointed out with particularity in the appendedclaims. The invention will, however, be more completely understood byreference to the following detailed description, which is accompanied bya drawing in which Figure 1 illustrates a signal generator structure inwhich the present invention may be employed and Figure 2 illustrates inenlarged section the lter network of the present invention, while Figure3 illustrates in section a modification of the embodiment of Figure 2.

Referring, now, to Figure 1 there is illustrated an ultra high frequencysignal generator tunable over a wide band of frequencies and having inits output circuit an attenuator including a Wollaston wire loop Rwwhich is slidable coaxially along the output cable to vary the outputamplitude. The signal generator shown in this figure may be operated inthe range between 600 and 1200 mc. Within the casing l) of the signalgenerator of Figure 1 is provided a high frequency oscillator includinga vacuum tube triode 2U whose plate P and grid G are connected at oneend to a tunable Lecher wire system 2|, 22 and at the other endcapacitively coupled by `means `ofnietallic plates 23,.-24 toxedinductorsng 25 and 26, respectively. Inductors 25 and 26 are each placedcoaxially within separate shielding chambers separated by wall |06. Thefilament F of vacuum tube 2i! has its terminals by-passed together bycondenser 21 for energy of the operating frequency. One conducto-r forsupplying energy to the lament F of the vacuum tube is comprised of ahollow tube 28 grounded at its far end while the other is comprised of alead extending through the interior of the hollow tube to asuitablesource of filament heating supply at the exterior of casing I0. Sliders29 serve to tune the plate and grid tuned circuits simultaneously whilethe slider 30 serves to tune the filament circuit. A suitable eccentric3| on a control-shaft extending to the exterior of the casing serves asa Vernier tuning element for the plate and grid circuits. Polarizingpotential for the plate P of the vacuum tube is provided through lead 32which is connected to one end of the conductor 2l. The output circuit ofthe signal generator within casing I includes an attenuator whichcomprises a hollow tube 3l slidable axially within a cylinder 3l'.Coaxially arranged within the tube 3l is a conductor 36, the inner endof which is connected to a Wollaston wire loop Rw, the other end of theloop Rw being connected through condenser 38 to the casing of the signalgenerator and also through cond-uctor 39 to terminal 48. Terminal 40 isalso by-passed to the casing by condenser 4i. The Wollaston wire Rw hasa resistance of the order of 'l5 ohms thus insuring an impedance matchbetween the coaxial output line formed by conductor 36 and hollow tube3l and the pick-up loop Rw. The amplitude 0f the output of the signalgenerator is varied by sliding tube 31 axially in and'out within thecylinder 3l.

Conductively connected to inductor 25 at a suitable point along itslength is a circuit including a Wollaston Wire resistor RW and a leadextending to the outside of casing l5] of the signal generator. A bridgecircuit including resistors l, 2, R2 and Wollaston wire Rw', togetherwith the microammeter li, together comprise a reference voltageindicator. It should be noted that the Wollaston wire Rw forms one armof the bridge, one of the terminals of which is tapped on to theinductance 25 to which the attenuator is also coupled. The ilow of highfrequency current through resistor Rw unbalances the bridge circuitcausing a deflection of microammeter 4. The output of the signalgenerator within casing l is preferably calibrated for a specicdeiieotion of the reference meter 4. The sensitivity of the referenceindicator is determined by an adjustment of resistor I4 in series withthe regulated power supply equipment 5.

It is very important that the only energy allowed to escape from thegenerator do so throughtheloutput conductorz-.36. Therefore, I

provide 10W-pass filter systems 33 each having high attenuation over thewide range f ultra high frequencies over which the generator is designedto operate. Thus leakage of ultra high frequency energy from the signalgenerator through the power supply leads and the leads for the measuringbridge is minimized. The iilter systems 33 each consist of a coaxialtransmission line having grounded outer conductors 34 and innerconductors 35 by-passed to the outer conductor at predetermined pointsalong its length by condensers 46. The details of construction of eachof the lter systems will be more readily understood by reference toFigure 2 wherein one of the filters is shown in an enlargedcross-section view.

Here the outer conductor 34 is shown as being broken at spaced points topermit the insertion of by-pass condensers 40 having their outer plates50 connected to the outer conductor 34 and, also, to the casing I0 ofthe signal generator. The inner plates of the condensers may take theform of comparatively heavy discs 5| having axial holes through whichthe inner conductor 35 passes. The discs are secured in place by setscrews 52 though other fastening means may be employed. Mica insulatingsheets 53 may `be provided on each side of the discs 5l to preventundesirable contact between the outer plates 50 and the inner plates 5|.The inner conductor 35 of the coaxial line is maintained in coaxialrelationship with the outer tubular conductor 34 by means of insulatingcentering washers 55 at the ends of each section of outer conductor 34.It will be noted that the outer conductor 34 at each end of each sectionis eiectively grounded to the casing l0 of the generator by means of theouter plates 50 of the condenser. In some extreme cases better shieldingmay be provided by enclosing the by-pass condensers 46 individually orthey may all be placed within extensions of the metal casing of thegenerator, as shown in Figure l.

The operation of the lter network, as described, may be illustrated bythe following example. It should be understood that no circuit lossesare considered. Assuming that the by-pass condensers 46 constitute ashort-circuit on the coaxial line, the reactanoe seen at the open end ofa coaxial line which is short-circuited at the far end is given by thefollowing equation:

X=Zo tan B1 B1 1 x A Xq t fifi?" pproxima e n.

degees short circuit ratio Ohms Ohms 90 M 0 0. 375 MSX.

The voltage attenuation ratio is approximately X/Xc. From the abovesample calculations it will be apparent that Z0 should be as large aspractical, that is, a high impedance line should be used. Furthermore,Xi. should be as small as practical, that is, the capacity of thecondensers 46 should be large in order to obtain the greatestattenuation. As the frequency is increased Xe becomes smaller which is ahelp, but this effect will not be realized if there is any inductancepresent in the condensers 46. It is therefore important that thecondensers be so arranged as to have a low inductance. This is assuredby the form of construction shown in Figure 2 where the condenser platesare rather massive and are symmetrically arranged with respect to theinner conductor 35 of the transmission line.

It Will be seen that an attenuation ratio greater than to 1 may be hadwith a single lter section over a 3 to 1 frequency range. Obviously, anumber of sections can be used to obtain greater attenuation. As amatter of fact, a number of sections are used in each of the leads ofthe signal generator of Figure l. Thus for three sections in series theminimum attenuation would be of the order of 1,000,000 to 1.Furthermore, sections of diiferent lengths may be employed to give adesired attenuation over a wider or narrower range of frequencies, asdesired.

It will be seen that at one end of the modification shown in Figure 2the inner conductor 35 is shown as arranged at right angles to the otherend of the conductor 35. This was desirable in the particularconstruction used in the signal generator of Figure 1 but is notessential for, obviously, the conductor 35 may as well be arranged inthe form of a single straight rod or shaft as shown in Figure 3. In somecircumstances it may be useful to utilize this form of constructionwhere it is essential that no signal energy is allowed to escape from asignal generator along a control shaft. Thus the shaft that controls theposition of eccentric 3l in Figure 1, or the shaft which may be providedto adjust the positions of sliders 29 and 30 may be conveniently besurrounded by a low-pass filter construction, such as shown in Figure 3.The shaft will be free to turn and may be electrically continuous butstill positively prevent the escape of high frequency energy along itslength. Thus any back lash which may be troublesome in the case ofinsulated couplers in a control shaft or the inherent weakness ofinsulating materials as control shafts is avoided.

While I have illustrated a particular embodiment of the presentinvention it should be clearly understood that it is not limited theretosince many modifications may be made in the several elements employedand in their arrangement and it is, therefore, contemplated by theappended claims to cover any such modications as fall in the sphere andscope of the invention.

I claim:

1. A band rejection lter including a coaxial transmission line having aninner conductor and an outer shell, said outer shell being divided intosections, parallel conductive plates arranged in pairs connected toadjacent ends of each of said sections and a conductive disc betweeneach of said pairs of plates and connected to said inner conductor, thecapacity between said conductive plates and said disc being so large asto present an extremely low reactance to the band of frequencies to berejected, the space between said conductive plates being such as topresent a high impedance to the band of frequencies to be rejected.

2. A band rejection iilter including a coaxial transmission line havingan inner conductor and an outer shell, said outer shell being dividedinto sections, parallel conductive plates arranged in pairs connected toadjacent ends of each of said sections and a conductive disc betweeneach of said pairs 0f plates and connected to said inner conductor andinsulating sheets interposed between the plates ci each of said pairs ofplates and said disc, the capacity between said conduc tive plates andsaid disc being so large as to present an extremely low reactance to theband of frequencies be rejected, the space between said conductiveplates being such as to present a high impedance to the band oifrequencies to be jected.

3. A band rejection filter including a coaxial transmission line havingan inner conductor and an outer shell, said outer shell being dividedinto sections, parallel conductive plates arranged in pairs connected toadjacent ends ci each oi sections and a conductive disc between each cisaid pairs of plates and connected to said inner conductor, said innerconductor passing through the center of each of said discs and saiddiscs being secured thereto by set screws, the capacity be tween saidconductive plates and said disc being so large as to present anextremely low reactance to the band ci frequencies to be rejected, theSpace between said conductive plates being such as to present a highimpedance t0 the band of ire quencies to be rejected.

fl. A band rejection lter including a coaxial transmission line havingan inner conductor and an outer shell, said outer shell being dividedinto sections, parallel conductive plates arranged in pairs connected toadjacent ends of each oi said sections and a conductive disc betweeneach of said pairs oi' plates and connected to said inner conductor,said inner conductor passing through the center of each of said discsand said discs being secured thereto by set screws and insulating sheetsinterposed between the plates of each of said pairs of plates and saiddisc, the capacity between said conductive plates and said disc being solarge as to present an extremely low reactance to the band offrequencies to be rejected, the space between said conductive platesbeing such as to present a high impedance to the band of irequencies tobe rejected.

5. A band rejection lter including a coaxial transmission line havingantinner conductor and an outer shell, said outer shell being dividedinto sections, parallel conductive plates arranged in pairs connected toadjacent ends of each of said sections, said parallel plates beingconnected along one edge to a conductive supporting plate, and aconductive disc between each of said pairs of plates and connected tosaid inner'conductor, said inner conductor passing through the center oieach `of said discs and said discs being secured thereto by set screws,the capacity between said conductive plates and said disc being so largeas to present an extremely low reactance to the band of frequencies tobe rejected, the space between said conductive plates being such as topresent a high impedance to the band of frequencies to be rejected.

6. A band rejection lter including a coaxial transmission line having aninner conductor and an outer shell, said outer shell being divided intosections, parallel conductive plates arranged in pairs connected toadjacent ends of each of said sections, said parallel plates beingconnected along one edge to a conductive supporting plate, and aconductive disc between each of said pairs of plates and connected tosaid inner conductor, said inner conductor passing through the center ofeach of said discs and said discs being secured thereto by set screwsand insulating sheets interposed between the plates of each oi saidpairs of plates and said disc, the capacity between said conductiveplates and said disc being so large as to present an extremely lowreactance to the band of frequencies to be rejected, the space betweensaid conductive plates being such as to present a nigh impedance to theband of frequencies to be rejected. Y

7. A band rejection fi ter including a coaxial transmission line havingan inner conductor and an outer shell, said outer shell being dividedinto sections, parallel conductive plates arranged in pairs connected toadjacent ends of each of said sections, said parallel plates beingconnected along one edge to a conductive supporting plate, and aconductive disc between each of said pairs of plates and connected tosaid inner conductor, said inner conductor passing through the center ofeach ci' said discs and said discs being secured thereto by set screwsand insulating sheets interposed between the plates of each of saidpairs of plates and said disc, said inner conductor being arranged forrotation Within said outer shell, the capacity between said conductiveplates and said disc being so large as to present an extremely lowreactance to the band or" frequencies to be rejected, the space betweensaid conductive plates being such as to present a high impedance to theband of frequencies to be rejected.

8. A band rejection iilter including a coaxial transmission line havingan inner conductor and an outer shell, said outer shell being dividedinto sections, each having a length of the order of onequarter or' themid-frequency oi said band, parallel conductive plates arranged in pairsconnected to adjacent ends of each of said sections and a conductivedisc between each of said pairs of plates and connected to said innerconductor, said inner conductor passing through the center of each ofsaid discs and said discs being secured thereto by set screws, thecapacity between said conductive plates and said disc being so large asto present an extremely low reactance to the band of frequencies to berejected.

9. A band rejection filter incluchng a coaxial transmission line havingan inner conductor and an outer shell, said outer shell being dividedinto sections, each having a length of the order of onequarter o themid-frequency of said band, parallel conductive plates arranged in pairsconnectedto adjacent ends of said sections, said parallel plates beingconnected along one edge to a conductive supporting plate, and aconductive disc between each of said pairs of plates and connected tosaid inner conductor, said inner conductor passing through the center ofeach of said discs and said discs being secured thereto by set screws,the capacity between said conductive plates and said disc being so largeas to present an extremely low reactance tothe band of frequencies to berejected.

10. A band rejection lter including a high impedance coaxialtransmission line having an inner conductor and an outer shell, saidouter shell being divided into sections, each having a length of theorder of one-quarter of the midfrequency of said `band,'parallelconductive plates arranged in pairs connected to adjacent ends of eachof said sections and a conductive disc between each of said pairs ofplates and connected to said inner conductor, the capacity between saiddiscs and said parallel conductive plates being such that atthe mid-bandfrequency the capacity therebetween presents a negligible impedance.

' RALPH W. GEORGE.

